This invention relates to precursors for and to new catalyst compositions useful for initiating and promoting polymerization of .alpha.-olefins and to a polymerization process employing such catalyst compositions.
It is well known that olefins such as ethylene, propylene and 1-butene in the presence of metallic catalysts, particularly the reaction products of organometallic compounds and transition metal compounds, can be polymerized to form substantially unbranched polymers of relatively high molecular weight. Typically such polymerizations are carried out at relatively low temperatures and pressures.
Among the methods of producing such linear olefin polymers, some of the most widely utilized are those described by Professor Karl Ziegler in U.S. Pat. Nos. 3,113,115 and 3,257,332. In these methods, the catalyst employed is obtained by admixing a compound of a transition metal of Groups IVB, VB, VIB and VIII of Mendeleev's Periodic Table of Elements with an organometallic compound. Generally, the halides, oxyhalides and alkoxides or esters of titanium, vanadium and zirconium are the most widely used transition metal compounds. Common examples of the organometallic compounds include the hydrides, alkyls and haloalkyls of aluminum, alkylaluminum halides, Grignard reagents, alkali metal aluminum hydrides, alkali metal borohydrides, alkali metal hydrides, alkaline earth metal hydrides and the like. Usually, polymerization is carried out in a reaction medium comprising an inert organic liquid, e.g. an aliphatic hydrocarbon, and the aforementioned catalyst. One or more olefins may be brought into contact with the reaction medium in any suitable manner. A molecular weight regulator, which is normally hydrogen, is usually present in the reaction vessel in order to suppress the formation of undesirable high molecular weight polymers.
Following polymerization, it is common to remove catalyst residues from the polymer by repeatedly treating the polymer with alcohol or other deactivating agent such as aqueous base. Such catalyst deactivation and/or removal procedures are expensive both in time and material consumed as well as the equipment required to carry out such treatment.
Furthermore, most of the aforementioned known catalyst systems are more efficient in preparing polyolefins in slurry (i.e., wherein the polymer is not dissolved in the carrier) than in solution (i.e., wherein the temperature is high enough to solubilize the polymer in the carrier). The lower efficiencies of such catalysts in solution polymerization is believed to be caused by the general tendency of such catalysts to become rapidly depleted or deactivated by the significantly higher temperatures that are normally employed in solution processes. In addition, processes involving the copolymerization of ethylene with higher .alpha.-olefins exhibit catalyst efficiencies significantly lower than ethylene homopolymerization processes.
Recently, catalysts having higher efficiencies have been disclosed, e.g., U.S. Pat. No. 3,392,159, U.S. Pat. No. 3,737,393, West German patent application No. 2,231,982 and British Pat. Nos. 1,305,610 and 1,358,437. While the increased efficiencies achieved by using these recent catalysts are significant, even higher efficiencies are desirable, particularly in copolymerization processes.
Even more recently, e.g. British Pat. No. 1,492,379, high efficiency catalysts have been employed which permit polymerization temperatures above 140.degree. C. Such high polymerization temperatures provide for reduced energy requirements in solution polymerization processes in that the closer polymerization temperature is to the boiling point of the polymerization solvent, the less energy is required in removing the solvent.
U.S. Pat. Nos. 4,250,286 and 4,269,733 disclose the preparation of a catalyst for polymerizing olefins in which the catalyst contains the product resulting from the admixture of a transition metal compound and a zinc compound.
It has now been discovered that such catalyst can be improved if the transition metal compound is first mixed with an oxygen-containing material before addition of the zinc compound. The catalysts subsequently prepared therefrom have higher initial rates of reaction and higher efficiencies than do those catalysts prepared without the oxygen-containing material.